In recent years gear changing mechanisms in conventional automobiles have changed in great part from manual transmissions to automatic transmissions of a a hydraulic nature. Even in many smaller trucks and buses, the mechanical transmission has been replaced.
However, in larger trucks the change from manual transmissions to those of the automatic variety has not been a completely successful effort for a number of reasons, a primary one being that the torque range transmitted by a hydraulic transmission is simply not adequate to cope with the great loads imposed thereon by the inertia of a large highway-type articulated trailer vehicle. Thus, in the great majority of the larger trucks manual transmissions will still be found.
Also as a direct result of the large and heavy loads imposed upon the drive trains of these trucks, it has been found necessary to use more than one transmission. The range of gear ratios needed to cope with the full spectrum of operating conditions of a large truck, from initiating movememnt from a stop up to maintaining speeds of 60 miles per hour and more, using a power plant of reasonable size, required incorporation of a gear changing mechanism having, commonly, 16 or 20 different gear ratios or speeds. For various reasons having nothing in particular to do with the present invention, the truck maufacturing industry almost universally accomplishes this speed change using two transmissions rather than attempting to build a single transmission having 16 gear changes.
As a result of this, the truck driver handling a large vehicle must cope with shifting two manual transmissions. The procedure, normally, is to place one transmission at one speed and then cause the other transmission to be shifted through its range, where upon it is then necessary to again shift the first and simultaneously down shift the second so that the procedure can be repeated. As will be recognized, this clearly necessitates the simultaneous shifting of two transmissions while the vehicle is in motion, a task which requires both of the driver's hands or the assistance of a second person in the vehicle cab. It should also be noted that this changing operation is not an infrequent occurrence; beginning from a standing stop, the driver finds it necessary in passing through city traffic to make 15 to 20 shifts in an interval of a few blocks.
It will be recognized that the situation outlined above presents a severe safety problem, in addition to the obvious requirements of strength and skill on the part of the truck driver.
Also forming a part of the background for this invention is the matter of present construction of manual transmissions. An example of a typical gear changing mechanism can be found in U.S. Pat. No. 1,372,227, Huggins in which yokes 19, 20 and 21 are coupled to movable gears within the transmission, the yokes being carried by rails 23, 24 and 25 which are moved by an external mechanism. In Huggins, they are moved electrically, and in the more usual situation, they are connected to a gear shift lever which is attached to the top of the transmission and which extends into the range of the driver's reach. Other examples of transmissions and techniques for changing the gear ratios therein will be found in U.S. Pat. Nos. 2,137,961, Vorech and 2,297,026, Sanford et al. A slightly different kind of transmission in which the gear changing forks are connected to a different form of linkage is shown in U.S. Pat. No. 2,244,092, Wheeler.
From these, it will be seen that the degree of motion available at the lower end of the gear shift lever is very small and must necessarily be so to limit the required extent of motion of the operator-handled end of the gear shift lever to a reasonable range of motion. This limited motion of the end of the shift lever inside the transmisson housing necessitates close spacing and limited motion of the rails, or other linkages, and the forks carried thereby, requiring that they be placed very close together and imposing other design limitations upon the transmission mechanism.
A further factor in connection with conventional manual transmissions relates to the speed with which a shift can be made. It would be ideal if a change from one gear to the next could be a substantially instantaneous one. With conventional manual transmissions, some time lapse is involved simply because the operator cannot move the mechanical elements with sufficient speed and precision. As a result, speed changes in the components involved occur and it is necessary to return the components to matching speeds, or reasonably close to matching speeds, before the shifting process can be completed. This necessitates the well-known "double-clutching" procedure necessary for many trucks.